PetaPixel

“Back in the days, when we were still capturing images on sheets of plastic, ISO (also known as ASA) was not a button on the back of a camera. It was a chemical property of the film. Some cameras could read the encoding on the film can and set the ISO accordingly. But sometimes you wanted to get more out of a film – to set it to a higher (or lower) ISO. This process is called pushing/puling the film, and if the camera you had could only do auto ISO decoding, you had to hack the film.”